Officials know of at least three such recent attempts, with one smuggler killed, another paralyzed and the third caught.

They don't know how many others have been made or succeeded or whether they've been tried outside Arizona.

The incidents are worrisome to federal officials. They believe more attempts are happening or will be. There's no agreement whether use of the small aluminum tubing aircraft represents a trend or a novelty.

Officials say progress in fortifying the border has spurred smugglers to rethink strategies.